Adhesives are ordinarily classified as thermosetting adhesives (permanently setting adhesives which are initially fusible or bondable but crosslink into a permanently infusible state), thermoplastic or hot melt adhesives (re-bondable adhesives which move from a bondable melt to a non-bondable or set state when cooled below their melting temperatures without crosslinking and becoming infusible) or pressure-sensitive adhesives which remain tacky (bondable) at ordinary room temperatures.
Thermosetting adhesives ordinarily develop high bond strengths but require relatively extensive periods of time, heat cycles and/or the elimination of volatiles to bond. Hot melt adhesives also can develop relatively high bond strengths, but require heat cycles to bond. Pressure sensitive adhesives are easily used but ordinarily develop considerably lower bond strengths than do adhesives of the other types. Furthermore, unlike the others, pressure sensitive adhesives tend to fail after a period of time under continuous load.
Despite the variety of adhesives which are available, there are a number of applications for which no really satisfactory bonding method is known. These include manufacturing operations involving bonding to certain plastics and electronic assemblies which are destroyed or ruined if subjected to high temperatures as well as bonding to substrates which are so massive that it is impractical to heat them, and where bonds obtained from pressure sensitive adhesives are of inadequate strength. Bonding accompanied by the elimination of volatiles is normally undesirable in such operations due to equipment, toxicity, fire, pollution, space and other considerations. Also, where surfaces to be adhered are relatively impervious, the escape of volatiles can result in blistering and poor adhesion. It would therefore be desirable to provide a technique by which strong dependable bonds could be formed without heating the substrate bonded and without involvement of volatiles.